This study investigated the effects of value similarity and procedural fairness of citizen participation on social acceptance of environmental policy decision in a case study of tram system in Neuss city, Germany. Value similarity between citizens and the authority is related to trust, particularly trust in authority, which is one of the significant factor on social acceptance. Procedural fairness and its antecedent factors with regard to citizen participation also affect social acceptance. However, it is uncertain that which factor (value similarity or procedural fairness) has s stronger effect on social acceptance. We argue that the importance of each factor depends on the participant’s involvement in the issue. In the sense, value similarity would be more influential on the acceptance for those who are highly involved in the issue, while procedural fairness would be more influential on the acceptance for those who are lower involved. We carried out a case study investigating tram system in the city centre of Neuss. In Neuss, the city center was too narrow and dangerous because the trams went through the main street. There was a big controversial debate of whether the tram should remain or be removed from there. After a decade of discussion - there were many opportunities of citizen participation and referendum during the years, the city mayor had decided the tram line to become a single track from double track on the section of the city center. Although the decision seemed a workable compromise, those who had strong opinions about the tram to remain or to be removed might not be wholly satisfied with the decision because their opinions were not perfectly reflected but half reflected. In the case, value similarity would play an important role, that is, people will accept the decision if they feel their value of policy for the city center and public transportation, which are the points at tram issue, is similar to the mayors’ policy. On the other hand, those who did not have strong opinion about the tram policy would weigh more on whether the procedure of reaching the decision was fair or not. To investigate how residents evaluate the decision process and to explore what factors influence people’s acceptance of the decision, a mail-out survey was conducted in Neuss using random sampling. Research hypotheses were a) Value similarity between respondents (citizens) and authority (the city mayor) would influence the acceptance of the decision mediated by the trust in authority, b) procedural fairness would influence the acceptance, and the main antecedent factors of procedural fairness would be information disclosure, representativeness and opportunity of voice, c) value similarity would have a stronger effect on the acceptance for those who are involved in the tram issue, while procedural fairness would have a stronger effect on the acceptance for those of lower involvement. // Results showed that hypotheses a) and b) were mostly supported, consistent with previous studies. Hypothesis c) was approximately supported. Both value similarity and procedural fairness determined the social acceptance. Value similarity with the city mayor had a stronger effect on the acceptance for those who are highly involved in the tram line issue than for those of lower involvement. However, both procedural fairness and trust in authority had strong effects on the acceptance even for those of higher involvement. Furthermore, not only trust in the mayor but also trust in the interested parties directed their activities for remaining/removing the tram correlated with acceptance of the decision particularly for those of higher involvement. The result also showed that even those who oppose to remain the tram tended to accept the decision if they evaluated the process as fair. The different roles of value similarity and procedural fairness will be discussed.